Cooling apparatus for high heat fluxes



Nov. 18, 1969 F. J. A. HUBER 3,478,820 Q COOLING APPARATUS FOR HIGH HEAT FLUXES Filed Sept. 28, 1967 4 Sheets-Sheet 1 IN VENT OR. P191902 4 19- ill/8&4

BY QM 7% g i Nov. 18, 1969 F. J. A. HUBER 3,478,820

COOLING APPARATUS FOR HIGH HEAT FLUXES Filed Sept. 28, 1967 4 Sheets-Sheet 2 /ill 2 INVENTOR. F169? 1?. 1/667 v Byg/ w-M Nov. 18, 1969 F. J. A. HUBER 3,478,820

COOLING APPARATUS FOR HIGH HEAT FLUXES Filed Sept. 28, 1967 4 Sheets-Sheet 5 Nov; 18, 1969 F. J. A. HUBER 3,478,820

COOLING APPARATUS FOR HIGH HEAT FLUXES Filed Sept. 28, 1967 4 Sheets-Sheet 4 error/veal;

United States Patent Oflice Patented Nov. 18, 1969 US. Cl. 165-47 3 Claims ABSTRACT OF THE DISCLOSURE The invention pertain-s to a structure which is inserted in the flow of a cooling channel, particularly as applied to a De Laval nozzle when the latter is subjected to severe heating conditions. The structure may take the form of inserts of metal plates arranged in one or several rows at the entrance to the cooling passage about the nozzle. These plates have a height as to extend part way across the width of the coolant channel. The plates are arranged in pairs and each plate is canted inwardly toward the other plate of the pair in the downstream direction. As the coolant is passing through a row of plates it is caused to twist around the plates which produces behind each pair of plates a pair of oppositely rotating swirls. in the coolant flow. These swirls move swiftly down the coolant channel while maintaining their helical streamlines. The swirls serve to plough the hot coolant layer away from the hot nozzle Wall and thus permit the cooler coolant from farther away to flow towards the wall and thus provide a stronger effect as far as the absorption of the heat by the coolant flow is concerned.

The counterrotating swirls can also be generated by streamwise tapes, alternately clockwise and counterclockwise, joined to each other at their sides and having alength in flow direction to cover one or several full turns of the twist.

Background of the invention The invention relates to apparatus for cooling surfaces which carry high heat fluxes such as hyperthermal expansion nozzles, water-cooled are electrodes and heat exchanger walls.

The invention significantly improves heat transfer effectiveness of a fluid moving past a solid wall. It has special application in De Laval nozzles which are limited in their use at high temperatures by severe heat fluxes in the throat. The throat regions of axisymmetric nozzles such as those used in rocket engines and hyperthermal wind tunnels are subjected to very high heat fluxes which must be absorbed by convective cooling. The simplest method of nozzle cooling is to force a liquid coolant (usually water) through an annular (backside) channel to absorb the heat fluxes. The cooling capability, however, is presently limited by the relatively poor heat transfer characteristics of the coolant film layer at the wall. For given heating conditions, the heat transfer effectiveness through the film increases with decreasing film thickness.

This invention involves the placement of swirl or vortex generator inserts in the annular cooling channel, thereby creating a sheet of parallel counterrotating vortices in the cooling fluid. The effect of the swirling coolant flow is two-fold. First, the swirls act as a plough on the film layer by sweeping the hot liquid layer from the wall; second, the centrifugal force field developed in the swirls forces the colder coolant (more dense fluid) radially outwardly in the swirls, displacing the warmer fluid of the film layer.

The scouring effects of swirling coolant, for example,

by the use of twisted metal tapes have been known for some time and employed in the coolant tubes of heat exchangers. The present application however differs in three main respects: (1) The counterrotating vortices are self-restrictive, thereby eliminating the requirement for a series of tubes to contain the individual vortices; (2) once initiated, the strong swirling flow is self-maintaining for a sufliciently long distance to be effective. Twisted tapes extending the length of the tubes which were required to maintain the swirl flow in previous applications of swirl cooling are thus also eliminated; (3) the vortices are created by low aspect ratio air foils or flat plates rather than twisted tapes running the length of the wall to be cooled.

The use of high pressure are heaters to raise total enthalpies of continuous gas flows has brought about the problem of high heat fluxes throughout the Wind tunnel. The heat fluxes experienced in the wall of the nozzle throat, often approach the limiting values which cause burnout, or failure of the nozzle wall.

Summary of the invention An object of the invention is to provide suflicient cooling of the nozzle wall to solve the problem of high heat fluxes.

Another object is to increase the heat transfer effectiveness between a temperature excited agent and a surface being subjected to the temperature of the agent.

Still another object is to reduce the thickness of the coolant film layer on a heated wall in order that the cooling effectiveness may be substantially increased.

A more general object is to reduce the obstructing effects of the heated coolant film layer at the surface of a hot wall and which prevents the colder fluid elements that are farther away from getting to the wall to be cooled.

These objects are attained in brief by the use of improved structure which constrains the coolant into a series of pairs of streams, each of which rotates in a different direction from the other stream of the pair, the reversely moving streams of helical shape being caused to contact the exterior surface of the nozzle and thus: plough or gouge the coolant film from that surface.

Other objects and features will be apparent as the specification is perused in connection with the accompanying drawings.

Brief description of the drawings FIG. 1 represents a longitudinal section of the expansion nozzle or end portion of an electric are heated airsupply for a hypersonic wind tunnel;

FIG. 2 is a sectional fragmentary view, greatly enlarged, of a section of the nozzle portion of the structure showing the manner in which the coolant moves through the coolant channel to contact the throat of the nozzle;

FIG. 3 is a fragmentary view, also broken away and in diagram, showing the entrance to the cooling channel and the position of the devices which cause the coolant to divide into a series of pairs of counterrotating swirls;

FIG. 4 is a greatly enlarged view of a typical number of the flat plates in their relative position within the entrance of the coolant channel and showing the manner in which the counterrotating swirls of the coolant are formed; this figure is taken at about the line 44 in FIG. 2;

FIG. 4A shows an arrangement of vortex generators which can be used in a channel where both walls have to be cooled or heated. Here, vortex generators are used at both walls to generate two layers of counterrotating vortices;

FIG. represents a diagrammatic fragmentary view of a section of the coolant entrance useful in explaining the optimum sizes of the air foils shown in FIGS. .2 and 3. This figure is taken at about the line 55 in FIG. 2, but is enlarged;

FIG. 6 is a perspective view (taken from a photograph) of a portion of a modified structure formed of tapes twisted alternately clockwise and counterclockwise. Shown are six tapes integrally joined at their sides to produce the rotating swirling motion in the cooling fluid and constituting a substitute for the flat plates; and

FIG. 7 represents a side view of the structure shown in FIG. 6 but on a larger scale.

Preferred embodiment of the invention Referring to FIG. 1, reference character 1 generally designates the nozzle section of a long, hollow structure which precedes a supersonic wind tunnel test section (not shown). These tunnels are employed for determining the optimum design and construction for airplanes and space vehicles in a fast-moving column of air which is usually heated to an extremely high temperature. The structure may comprise a pair of coaxial tubes in tandem arrangement (not shown) in which a long electric arc is stabilized along their common centerline to heat the air and the heated air is then passed through a De Laval nozzle shown at 2 in the figure in order to raise the air speed up to the desired range as it passes into the test section. The nozzle throat is formed of a thin cylinder of copper integrally joined at one end to the entrance section 3 of slightly thicker dimension and tapered outwardly to form the one end of a cylindrical cathode of an arc heater (not shown). The anode (not shown) of the arc heater is secured to the remote end of the cathode. The other or right-hand end of the nozzle throat is integrally joined to the expansion section 4 which is slightly thicker than the nozzle throat and is of considerably greater length than the entrance section 3. The nozzle liner is surrounded by a cylindrical metal piece 5 and the latter is contained in a separate metal piece 6, The dividing line 7 between the cylinder 5 and the metal piece 6 is of a shape so designed that one element can be removed longitudinally with respect to the other when the screws 8 are loosened and both elements may be removed in order to permit the nozzle liner to be taken out and replaced if necessary. The metal piece 6 has a cylindrical exterior which extends the full length of the nozzle portion. A cylindrical cahing 9 surrounds metal piece 6 and is provided at the righthand end with a thick plate or flanged member 10. Bolts 11 are provided in the flange member for securing that member to a heavy ring-like plate 12 forming part of the nozzle extension to the test section of the wind tunnel. The plate is provided with a radially extending opening 13. A bent conduit 14 is inserted into this opening to accept the cooling water from the nozzle.

At the opposite end, the casing 9 is welded or otherwise secured to one edge of a metal ring 15. The latter supports a vertical conduit 16 for receiving a pair of inlets 17 for the cooling water. The other edge of the ring is carried on a heavy flange member 18 which is bolted as indicated at 19 to abut a flange 20. The latter constitutes the end support of a hallow cathode structure throughwhich air is passed and heated by an arc discharge (not shown) and then is driven toward the throat 2 of the nozzle section.

As was described hereinbefore, the ring 15 is supported only at one end on the casing 9. This would leave an annular space 21 between the outer surface of the metal piece 6 and the inner surface of the ring member 15. The flange member 18 has a circular notch at 22 in communication with this space. It will be noted that there is an annular space 23 between the nozzle entrance liner 3, the throat of the nozzle 2 and the nozzle expansion line 4 on the one hand, and the adjacent metal members 5 and 6, respectively, on the other hand. The wide surface of the left-hand end of the metal piece 6 is cut short as indicated at 24 to provide a passageway between the circular notch 22 and the left-hand end of the space 23. Likewise, the right-hand end of metal piece 6 is also cut short as indicated at 25 to provide a passageway between the space 23 and the transverse opening 13. Thus, there is a continuous passageway from the inlet 17 through 21, 22, 24 and 23, 25 to the outlet opening 13. This passageway follows the exterior surface of the nozzle throat liner 2. A coolant such as water is sent under pressure through the passageway in order to maintain the throat 2 at all times below its maximum permissible operating temperature.

I have discovered, in accordance with the principle of my invention, that the effectiveness of this cooling layer of water passing through the long passageway can be greatly increased by causing the fluid to move in the form of vortices and to cause these vortices to counterrotate with respect to one another, The effect of the swirled coolant flow is two-fold. First, the swirls act as a plough on the film layer by sweeping the relatively hot fluid layer from the wall; secondly, the centrifugal force developed in the swirls forces the colder and therefore more dense coolant fluid radially outwardly, thus, displacing the warmer fluid of the film layer on the nozzle liner. The counterrotating vortices are self-restricted, thereby eliminating the requirement for a series of tubes to contain the individual vortices and once initiated the swirling flow in self-maintaining for a long enough distance to be effective through the critical heating region around the throat area.

The counterrotating vortices are attained in various ways. The preferred structure is shown in FIGS. 2, 3, 4 and 5. In FIG. 3 low aspect ratio air foils or flat plate inserts 26 are provided in the entrance portion 27 of the annular channel. The flow area is the greatest at this point and the dynamic pressure is relatively low. This, in turn, minimizes any pressure loss due to the inserts. In FIG. 1, the position of these air foils or inserts is indicated by the three uprights 26. They are constructed of metal and of suflicient span (or height) to extend about half-way across the annular space and the chord length of each may be equal to or longer than its span. The inserts may be affixed edgewise within the space by means of a flange (not shown) along one edge screwed or otherwise fastened to the metal member 5. These edges may be ground to accord with the configuration of the round surface. The inserts, of which three typical circumferential rows are shown, are arranged in pairs with the trailing edges of each pair canted toward one another as seen in FIG. 3. Each pair of inserts cooperates with the pair of inserts of the rows following downstream as to produce a pair of vortices in the coolant flow as indicated in FIGS. 2 and 3. The angle of incidence of these plates is increased at the downstream rows as can be seen in FIG. 3 so as to impart additional angular momentum to the generated swirls. It will be found that these vortices will rotate above and below their central axes indicated by the dot-dash line 28 in FIGS. 2 and 3 in the opposite direction with respect to the adjacent vortices as they move down toward the throat. To further assure that the vortices will fill the coolant channel thereby achieving maximum mixing, the channel height is made approximately equal to the width of a single vortex cell. This geometry results in an approximately square cross section for each vortex as indicated by the dot-dash lines 29 in FIG. 5.

Another geometric characteristic that is unique, as applied particularly to a De Laval nozzle, is that the channel height should decrease proportional to the channel circumference in the converging portion of the nozzle. The result is that the square cross section of the vortex cell is maintained. Moreover, the moment of inertia of each vortex about its rotational axis will decrease as the fluid converges toward the throat. In accordance with the law of conservation of angular momentum (the product of the moment of inertia and the angular velocity must remain constant), the angular velocity will increase inversely proportional to the channel height. This increasing angular velocity will then achieve its largest value at the nozzle throat where the highest heat fluxes are encountered.

An additional advantage of the increasing angular velocity is the hydrodynamic stability it affords the vortex which is otherwise subject to rapid decay due to friction. The channel geometry downstream of the nozzle throat is not so critical since heat fluxes experienced at this region are much lower than in the throat. In all probability, a vortex flow generated upstream would not be sustained very long at a high strength in the diverging portion of the nozzle where the flow area is increased. It is necessary only to maintain a flow area such that the axial velocity of the coolant is high enough to provide sufficient heat transfer. I

Thus, the formation of the vortices closely adjacent to one another and rotating in opposite directions, by the use of the inserts, plough or scour the heated coolant film from the exterior surfaces of the conically shaped portions 3 and 4 of the nozzle in addition to the throat portion 2 where the heat flux is the greatest. The removal of this coolant film permits the colder water or other coolant to flow toward the metal of the throat directly and thus increase the cooling effectiveness at the severe heating area.

While I do not wish to be limited to any theory as to the-formation of these vortices by the angularly related inserts acting on the fast-moving coolant flow, I believe they are produced by the difference in pressure between the sides of each insert which induce a flow from the over pressure surface toward the other surface around the longitudinal edge. The strength of this flow, which is a vortex, is measured in terms of circulation. This circulation or vortex flow cannot end abruptly at the outer edge of the insertbut must continue downstream in the form of a trailing tip vortex, The direction in which the inserts are canted determines the direction of flow of the vortices, so that when the inserts are directed inwardly toward one another, the vortices are caused to rotate adjacent one another but in opposite directions, thus eliminating any fluid friction between them. These vortices are so pronounced that they remain in these positions for a considerable distance beyond the throat, notwithstanding the decreasing thickness of the channel space. Due to the fact that the swirls are counterrotating, the friction between adjacent swirls is eliminated, thus increasing their lifetime of effectiveness. Thus the exterior surface of the conically shaped copper linings 3, 4, and more especially the throat portion 2, where the heat flux is the greatest, receive the greatest amount of the cooling etfect from the swirling motion in the coolant which is now able to flow much closer to the metal surface.

Still another method of obtaining the counterrotating vortex etfect is shown in FIGS. 6 and 7. These fragmentary views illustrate the plan view (slightly on a perspective basis) and the side view (considerably enlarged) of a structure which when bent to a cylindrical shape and properly fitted within the cylindrical coolant space 27 in the same position as the plate inserts, will provide the counterrotating vortices of the water or other coolant and the convolutions of which are wound fairly tight so as to persist in this shape for a distance well beyond the nozzle region. It is preferred that the vortex-producing structure be formed of copper or other material which may be readily bent and-cut to thecylindrical shape required by the upper end of the member 3. However, it will be understood that the drawings show this member after it has been molded and in a flat form prior to the bending and fitting operation mentioned. It is of a thickness as to fit snugly across the full height of the entrance of the channel.

This form of vortex-producing structure consists of parallel arranged tapes of metal or other sutficiently strong material that are alternately twisted clockwise and counterclockwise and aligned in the coolant flow direction. These tapes may have a length to cover one or several full turns of twist so that a sufiiciently strong swirling motion is imparted to the coolant flow before leaving this structure. To start with, the untwisted tapes may be wider by a factor of approximately 1.5 than the height of the cooling channel. The twisted tape may then be trimmed such that its axially projected area becomes the same as the segment of the cooling channel into which it has to fit in a side-by-side arrangement with the other tapes. The longitudinal alignment of the tapes is such that the plane between adjacent tapes forms a mirroring plane thus allowing the two tapes to be joined at their common meeting line with solder or adhesive. The twisted tape arrangement shown in FIGURES 6 and 7 has a streamwise length of one full turn of twist of the tapes. This structure can be fitted in the space as indicated by 27 in FIG. 1.

Thus, the structure shown and described in connection with FIGS. 6 and 7 provides the same advantages as the inserts 26 (FIGS. 2 to 4) in serving to plough or gouge the heated coolant film from the exterior surface of the copper members 2, 3 and 4 to give full access of the coolant to the meal over which the coolant travels. The improved vortex-producing devices of FIGS. .2 and 6 have been tested from the standpoint of etfectiveness in producing these tightly wound counterrotating vortices and the increase in cooling eflfect has been found to be not less than 25% due to the removal of the coated film from the exterior of the nozzle portions as explained hereinbefore.

As in the case of the flat plates 26, the coolant is caused to separate into many helical paths indicated at 30 in FIGS. 6 and 7 to form pairs of counterrotating vortices. These paths are broadly indicated by the light full lines 30 in both figures where they occur in full view of the observer but as dotted lines where they are constrained by the shape of the tape structure to be hidden from view. These vortices are sufificiently strong to carry well past the throat 2 and thus cause the heated coolant film at this position to be dislodged so as to allow the cooler water directly to strike the throat as it passes to the outlet 14 (FIG. 1).

While I have described my invention in connection with a high temperature, high pressure air flow, produced by an electric arc heater, that is being forced through a symmetrical De Laval nozzle into the cylindrical test section of a hypersonic wind tunnel thereby requiring a throat structure 2, it will be understood that this invention is not limited to apparatus of this type.

The invention may be applied to cases where both walls are to be cooled. In this case, the center surface through the middle of the cooling channel height has to be made a mirroring surface for the flow pattern with a layer of counterrotating vortices on either side and the vortices rotating in such directions that each neighboring vortex is counterrotating. A section taken at the line 44 in FIG. 2., and assuming that additional inserts 26' are secured to the exterior surface of nozzle portion 3 as well as to the interior surface of the member 5, would take the form shown in FIG. 4A. A description of this figure and its relationship with the rest of the structure is similar to that set forth in connection with FIG. 4, both being taken when looking in the flow direction.

Since the generated vortex flow pattern increases the heat transfer effectiveness, it can be applied to heating as well as cooling. Furthermore, since the mechanism of heat transfer is similar to that of mass transfer, these vortex flow inserts can also be used for evaporating fluids or drying of paint coats, etc.

The structure may be applied to increase heat transfer effectiveness of any type of surface, flat or cylindrical. There are no moving parts and it is easy to manufacture and install in conventional (straight flow) cooling and heating channels. The use of the counter-flow adjacently positioned vortices attained in the manner stated, increases the allowable heat flux at a given maximum wall temperature and extends the thermal capability of the high heat flux surfaces such as hyperthermal nozzle throats, water-cooled arc electrodes and heat exchanger walls.

While a certain specific embodiment has been described, it is obvious that numerous changes may be made without departing from the general principles and scope of the invention.

I claim:

1. Apparatus for bringing a coolant to a heated wall through a channel, said apparatus including means constituted of directive surfaces extending at least part way across the coolant channel and positioned immediately prior to the heated wall, causing the coolant to emerge from said surfaces in the downstream direction as a series of counter-rotating vortices which contact the wall and serve to plough the heated coolant layer away from the wall whereby the colder coolant farther away from the wall is permitted to replace the heated coolant in order to absorb the heat flux from the wall, said directive surfaces being constituted of pairs of relatively short plates positioned edgewise in the coolant channel, the plates of each pair being canted inwardly toward one another in the downstream direction whereby the coolant in passing between the respective pairs in said rows is caused to divide itself into a series of counter-rotating vortices moving adjacently parallel to one another downstream and without interference in order to contact the heated wall and to cause the heated coolant layer to be swept away from the wall and permit the colder coolant to flow to the wall.

2. Apparatus according to claim 1 and in which the canted short plates occur as a plurality of rows in the downstream direction, and the degree of cant becoming progressively greater from row to row in the flow direction whereby additional angular momentum is imparted to the generated swirls.

3. Apparatus for bringing a coolant to a heated wall through a channel, said apparatus including means constituted of directive surfaces extending at least part way across the coolant channel and positioned immediately prior to the heated wall, causing the coolant to emerge from said surfaces in the downstream direction as a series of counter-rotating vortices which contact the wall and serve to plough the heated coolant layer away from the wall whereby the colder coolant farther away from the wall is permitted to replace the heated coolant in order to absorb the heat flux from the wall, said directive surfaces being constituted of pairs of relatively short plates positioned edgewise in the coolant channel, the pairs of plates being positioned in a plurality of rows in the downstream direction, the plates of each pair within each row being canted inwardly toward one another, the degree of cant becoming greater in the downstream direction whereby the coolant in passing between the respective pairs in said rows is caused to divide itself into a series of counterrotating vortices moving adjacently parallel to one another downstream and without interference with one another, said vortices serving to plough the heated coolant layer away from the wall and permit the colder coolant from farther away to absorb the heat flux from the heated wall.

References Cited UNITED STATES PATENTS 3,027,143 3/1962 Furgerson et al 165156 3,205,147 9/1965 Foure et al. 165109 3,378,453 4/1968 Gorker 165-109 ROBERT A. OLEARY, Primary Examiner C. SUKAL, Assistant Examiner US. Cl. X.R. 165l09, 134 

